I. Field of the Invention
This invention relates to a method for obtaining calibrated image data for a target object using a tomographic apparatus. The calibrated image data is corrected to compensate for the use of different collimator widths to obtain the calibrated image data. The calibrated image data, which have reduced systematic noise, are used to reconstruct a tomographic image of the target object.
II. Background Information
Conventional methods for obtaining image data for a target object with a tomographic apparatus include steps for calibrating the image data of the target object against image data obtained for a reference object. This calibration is performed in order to eliminate, from the image data for the target object, non-object-intrinsic information or inaccuracies which may be contained within the image data as a result of features of the tomographic apparatus and techniques utilized to obtain the image data using the tomographic apparatus.
Typically, a tomographic apparatus includes a radiation source for generating radiation beams, a collimator for collimating the radiation beams to restrict the beams to a slice, and a chamber through which these radiation beams are directed. The collimator typically has a plurality of blades for use in setting a width for the collimator. The chamber of the tomographic apparatus includes a portion specifically adapted for receipt of an object. The tomographic apparatus further includes a plurality of individual detectors located opposite the radiation source for detecting radiation emerging from the chamber at various points, a data aquisition unit and a memory for storing data.
Image data for the target object may be obtained by placing the target object within the portion of the chamber adapted for receipt of an object, setting the width of the collimator to a predetermined scan-of-interest width appropriate to the width of the portion or slice of the target object which is of interest, and subsequently subjecting the target object to radiation from the radiation source. The radiation from the radiation source will pass through the object and emerge from the chamber so as to be detected by the detectors positioned at various points opposite the radiation source. The intensity of each radiation beam emerging from the chamber will vary in accordance with the composition of the target object at the points through which each radiation beam has passed. Each detector responds to the emerging radiation detected at a given point by generating a response signal for the radiation detected at that point. The response signals are transferred to the data acquisition unit of the tomographic apparatus which generates image data for the target object. The image data are stored in the memory.
For various reasons, however, the composition of the target is not accurately reflected by the emerging radiation which is detected by the detectors when the target object is placed in the chamber portion and subjected to radiation, i.e., scanned. Accordingly, the image data generated when the target object is scanned is inaccurate and not ideal for reconstructing a tomographic image of the target object.
One reason why the image data obtained by scanning the target object is not ideal is that a portion of the radiation detected by a detector is scattered radiation. Most tomographic apparatus base the reconstruction of images on the detection of transmitted radiation only, and not also on scattered radiation. Transmitted radiation is attenuated as it traverses the target object according to composition and density of the target object, and emerges from the chamber at a position opposite the radiation source. Scattered radiation is deflected by the target object and emerges from the chamber at some position other than opposite the radiation source. Radiation may be deflected at more than one point within the target object. Accordingly, a detector may receive both transmitted radiation, which follows a straight path from the radiation source, and scattered radiation, which follows a single or multiply deflected path from the radiation source. The detectors are unable to distinguish between transmitted and scattered radiation. As a result, scattered radiation causes the generation of image data which is inaccurately indicative of target object composition and density. The typical target object scatters at least some measure of radiation. Anti-scattering devices may reduce, but not eliminate, the amount of scattered radiation that reaches the detectors.
Another reason for inaccurate image data may be that the radiation from the radiation source is polychromatic X-ray radiation comprising a continuum of energy levels. The generation of image data and reconstruction of tomographic images is based on the idealization of monochromatic radiation having a given discrete energy level. Typical tomographic apparatus include radiation sources which, however, emit polychromatic X-ray radiation, and which, therefore, produce inaccurate images from the image data obtained from a scan of the target object. Other reasons for inaccurate image data such as, for example, effects caused by radiation generated by non-point sources and differences between individual detectors may also exist.
To mitigate against the effects of scattering, polychromaticity, and other undesirable effects, as those effects appear in the image data obtained when scanning the target object, conventional methods for obtaining image data, as stated above, also scan a reference object, and calibrate the image data obtained for the target object against data obtained for the reference object. The reference object, which is chosen and scanned to obtain data for use in calibrating the data for the target object, typically has physical characteristics essentially similar to the target object in so far as the characteristics of the target object and reference object are related to undesirable effects caused by, for example, scattering and polychromaticity. The collimator of the tomographic apparatus is set to a predetermined reference object collimator width for the reference object scan.
The target object may be, for example, a human patient or a portion of a human patient, the patient being scanned in order to produce a tomographic image for diagnostic use. In such a case, the reference object selected for use in calibrating the image data for the target object is typically a cylinder of water. The water cylinder is chosen having an overall external size similar to the size of the patient, i.e., to the "field size" for the target object. The physical characteristics of water which relate to scattering and polychromaticity are substantially similar to those of the human patient. Both the human patient and the water cylinder demonstrate substantially similar responses to polychromatic X-ray radiation, and both scatter radiation in a similar manner.
Although the water cylinder is similar to the human patient in regard to overall external size, and to physical characteristics relating to, for example, scattering and polychromaticity, the water cylinder is distinct from the human patient in regard to external shape and the size and shape of internal organs. The water cylinder is devoid of internal components.
Accordingly, data obtained by scanning the water cylinder primarily capture undesirable effects such as scattering and polychromaticity, for an object of its size, but does not capture external shape and internal features. Data obtained by scanning the human patient capture the external and internal features of the patient and undesirable effects.
Since the data obtained by scanning the water cylinder and the data obtained by scanning the human patient differ only in that the data for the human patient scan capture external and internal physical features of the human patient (in addition to the undesirable effects also captured by the water cylinder data), the human patient data are differenced from the water cylinder scan data. This differencing eliminates, or at least minimizes, the scattering and polychromatic effects from the human patient data. The resulting difference data set, which is representative of a scan of the human patient, free of undesirable effects, is referred to as the calibrated image data set of the human patient.
The water cylinder used when obtaining calibrated image data for a human patient is typically of uniform dimensions (wall thickness and diameter) throughout. The data generated for the cylinder are adjusted to achieve a substantially flat or equivalent set of signal responses from the detectors, i.e., a flat profile. In order to achieve a flat profile, a compensation object, typically an aluminum wedge, which is concave where the water cylinder is convex, may be inserted in the tomographic apparatus chamber at a location between the radiation source and the portion of the chamber where the human patient would be placed.
Data obtained when the reference object, e.g., a water cylinder, is scanned and image data obtained when the target object, e.g., a human patient, is scanned are each processed to correct for detector-to-detector variations and variations of the radiation output by the source, and data obtained is processed to obtain logarithms of all data values. Specifically, a water cylinder-calibrated image data set is obtained by differencing the processed water cylinder data and processed patient scan data. The natural logarithms of this differenced data set are actually used to reconstruct a tomographic image of the patient. Notice that effects caused by the compensation object, i.e., aluminum wedge, mentioned in the previous paragraph cancel when the differencing operation is performed. The water cylinder data, the image data, and the water cylinder-calibrated image data all may be stored in the memory of the tomographic apparatus. The tomographic apparatus also comprises a data processing unit. The processing steps mentioned above are performed by the data processing unit.
The amount of radiation generated by the radiation source which reaches the chamber of the tomographic apparatus is proportional to the width of the collimator. Accordingly, the amount of radiation emerging from the chamber, and the response signals generated in response thereto are also proportional to the width of the collimator. The collimator width for the essentially featureless reference object, e.g., the water cylinder, is chosen to maximize the signal-to-noise ratio, where here noise refers to statistical (i.e. random) noise, in the scan data for the water cylinder. Water significantly attenuates radiation beams passing through the chamber of the tomographic apparatus. Such significant attenuation tends to make the signal-to-noise ratio low. Hence, to keep the signal-to-noise ratio as high as possible in the reconstructed tomographic images, the collimator width for the reference object scan is made as large as possible to allow the greatest flow of radiation through the chamber.
The same argument can be applied to the image data scan to suggest the widest possible collimator width for the target object, that is, human patient scan data. However, for human patient scans, other considerations are of greater importance than the consideration with respect to statistical noise. The maximum collimator width is not necessarily ideal for a slice of the human patient. The collimator width for the target object, i.e., the human patient, is preferably chosen to maximize diagnostic information and minimize patient irradiation. A narrow slice in the human patient, and correspondingly a small, possibly even the minimum, collimator width, may be necessary to image and localize a small internal structure, e.g., a tumor, in the human patient. On the other hand, given pertinent diagnostic considerations, a wide slice in the human patient, and correspondingly a wide, possibly even the maximum, collimator width, may be called for. Thus, the reasons for picking collimator slice widths for the reference object scan and the target object scan are not the same, and may therefore lead to a situation wherein different collimator slice widths are preferably chosen for the water cylinder and human patient scans.
As stated above, the reference object, i.e., water cylinder scan, was introduced to mitigate undesirable effects in the target object, i.e., human patient image data such as scattered radiation and polychromaticity, and ordinary processing, as mentioned previously, mitigates these undesirable effects even if the reference object and target object scans are performed using different collimator widths. However, if different collimator widths are used, a previously unmentioned undersirable effect may enter the reconstructed tomographic image.
As discussed above, the response signal generated by the detectors is proportional to the width of the collimator. But the width of the collimator may not be a constant width along the full length of the collimator. That is, the width of the collimator may fluctuate about its nominal width due to, for example, bowing of the blades that define the collimator or machining marks or nicks in these blades. And each collimator width setting has its own unique pattern of width fluctuation along the length of the collimator. Therefore, the signal response of the detectors may reflect this collimator width fluctuation. If different collimator widths are used for the reference scan and target scan, then the data sets of each scan reflect different collimator fluctuation patterns. These patterns do not cancel when forming the water cylinder-calibrated human patient data set by differencing. Hence, undesirable effects are in the water cylinder-calibrated patient image data and may appear in the reconstructed tomographic image.
In summary, a problem exists in that calibration of the image data for a human patient scan, obtained when the collimator has a given width, using data for a water cylinder scan, obtained when the collimator has another given width, minimizes scattering and polychromatic effects, yet use of the different collimator widths to obtain the data introduces other undesired effects in the reconstructed tomographic image.